In order to regulate the brightness of light emitting diodes (LEDs) over a wide range, variation of the LED current operating the LEDs (current level modulation) is unsuitable since, at low and high currents, the brightness of the LEDs can deviate from a linear current/brightness ratio that is otherwise readily fulfilled, and the LEDs can, moreover, be damaged. Therefore, LEDs are usually operated by means of a pulse width modulation (PWM), in which the LEDs are operated with a constant current I for a predetermined switched-on duration t1 (also designated as switched-on time) and thereafter are switched to be currentless for a predetermined switched-off duration t2 (also designated as switched-off time). A duty ratio t1/T having a period duration T=(t1+t2) in this case corresponds approximately to a desired dimming level. If the period duration T=(t1+t2) is chosen to be too long (typically longer than 10 ms), this PWM dimming method can be perceived by a human observer by virtue of flicker. The current intensity or the current I, integrated over the switched-on duration t1, defines the light energy or quantity of light generated in the period T.
PWM driving or the PWM dimming method makes dynamic demands of a current source used. In the ideal case, a constant current is made available for the switched-on duration t1, and no current flows during the switched-off duration t2 (that is to say that the switch-over process is carried out negligibly fast). Only then is the duty ratio t1/(t1+t2) directly proportional to an effective brightness or intensity of the LED(s). In the case of current sources having a high rise time constant t_PSU (that is to say that duration required by the current source to raise its current value from the switched-off state to a predefined proportion of the nominal value) and low dimming levels, this is no longer ensured, however, such that the dimming behavior then deviates from the linear behavior.
In the case of (color) mixing of light from different-colored LEDs with different required intensity ratios or individual switched-on durations (e.g. t1i for i different LEDs), in order to be able to set a certain cumulative color locus, it is important to keep the intensity ratio or brightness ratio between the different LEDs at a specific value. In the case of an excessively low dimming level (ratio of the present current intensity to a maximum current intensity), the brightness ratio e.g. between two different-colored LEDs is no longer proportional to t1a/t1b, however, and the effective mixed color changes appreciably.
Consequently, the minimum dimming level usable in practice has hitherto essentially been defined by the rise time constant t_PSU or the speed of the current source and the period duration T. In practice, therefore, period durations T of approximately 0.1 ms are not undershot, which disadvantageously limits the dimming level. The dimming by means of the pulse width modulation is consequently used, in particular, for period durations T in a range of 10 ms>T>0.1 ms. Flicker is no longer perceptible for such period durations T.
One possibility for tackling these disadvantages is to use fast current sources, but the latter are comparatively expensive. It is also possible to accept flicker at low dimming levels and/or color deviations at low dimming levels, but this is deemed to be disadvantageous by a user.